patch 8.0.0693: no terminal emulator support

Problem:    No terminal emulator support.  Cannot properly run commands in the
            GUI.  Cannot run a job interactively with an ssh connection.
Solution:   Very early implementation of the :terminal command.  Includes
            libvterm converted to ANSI C.  Many parts still missing.
diff --git a/src/libvterm/t/46screen_extent.test b/src/libvterm/t/46screen_extent.test
new file mode 100644
index 0000000..a126cec
--- /dev/null
+++ b/src/libvterm/t/46screen_extent.test
@@ -0,0 +1,11 @@
+INIT
+WANTSCREEN 
+
+!Bold extent
+RESET
+PUSH "AB\e[1mCD\e[mE"
+  ?screen_attrs_extent 0,0 = 0,0-1,1
+  ?screen_attrs_extent 0,1 = 0,0-1,1
+  ?screen_attrs_extent 0,2 = 0,2-1,3
+  ?screen_attrs_extent 0,3 = 0,2-1,3
+  ?screen_attrs_extent 0,4 = 0,4-1,79